The air battery is a nonaqueous battery using air (oxygen) as a cathode active material, and has advantages such as having a high energy density, and facilitating downsizing and weight saving. In such an air battery, when a metal Li is used for an anode active material for example, the following reactions (1) to (4) are mainly generated.
[Chemical Formula 1]
Discharge TimeANODE: 2Li→2Li++2e−  (1)AIR CATHODE: 2Li++2e−+O2→Li2O2  (2)
A little amount of Li2O may sometimes be generated other than Li2O2.
Charge TimeANODE: 2Li++2e−→2Li  (3)AIR CATHODE: Li2O2→2Li++2e−+O2  (4)
Various researches have been conducted conventionally in order to maximize the advantages of air batteries. For example, Patent Document 1 discloses a nonaqueous electrolyte air battery using a specific ambient temperature molten salt as a nonaqueous electrolyte. This air battery is to prevent the volatilization of a solvent by using the specific ambient temperature molten salt, and thereby improves discharged capacity at high temperature and the discharged capacity after being stored under a high humidity. Patent Document 2 discloses a nonaqueous electrolyte battery comprising a cathode which uses a carbonaceous substance having a specific porous capacity. This focuses on the specific porous capacity of the carbonaceous substance and the like and intends to increase the high capacity of the battery. Accordingly, approaches to improve functionality of constitutional member of a battery have been the main stream of the conventional researches.
However, the air battery has a problem that the volume of electrodes (air cathode and anode) changes significantly according to a discharge or a discharge and charge, which causes a shortage of liquid electrolyte. To specifically explain the above-mentioned reaction, at the time of discharge, Li elutes out as a Li ion at the anode (reaction (1)), and lithium oxide is precipitated at the air cathode (reaction (2)). At this time, since a degree in density of lithium oxide (Li2O2) is larger than that of Li, a shrinkage of 35% in volume ratio as the whole electrodes is caused. As a result, there has been a problem of increasing the internal resistance caused by a part of air cathode and the like not soaking into the liquid electrolyte because of a shortage in the liquid electrolyte amount occurred at end phase of the discharge. Further, when a carbon material such as graphite is used as an anode active material other than a metal Li, a volume change at an anode is less, but Li2O2 is generated at an air cathode and a liquid electrolyte in the air cathode is pushed out. Consequently, the liquid electrolyte is transferred to a gap or the like in the battery, and the liquid electrolyte is less likely to transfer back to the air cathode after the dissolution of Li2O2 caused at the charge. As a result, the shortage in the liquid electrolyte amount is caused and a problem of increasing an internal resistance is caused. Therefore, an air battery which can restrain the internal resistance caused by the shortage of liquid electrolyte from increasing has been called for.
On the other hand, an encapsulated type air battery in which a gas such as oxygen is sealed in the battery case has been known. For example, Patent Document 3 discloses an encapsulated type oxide-lithium secondary battery, wherein the gas including pressured oxygen is sealed in an exterior part of the air battery. This is to restrain the intrusion of moisture in air to the battery by making the oxide-lithium secondary battery as an encapsulated type, and thereby to improve the storage properties of the battery or the cycle life of discharge and charge. Nonetheless, such oxide-lithium secondary battery has the following problems.
As shown in the above-illustrated reaction (2), an air cathode needs oxygen and the density of oxygen dissolved in a liquid electrolyte decreases by the reaction at a discharge. In case of the above-mentioned oxygen-lithium secondary battery, there has been a problem in maintaining the density of dissolved oxygen high and the high-rate discharge has been difficult to carry out. In the above-mentioned oxygen-lithium secondary battery, the pressured oxygen is sealed therein. Thus, the oxygen is more likely to dissolve in the liquid electrolyte compare to the case when no pressure is applied to the oxygen. Nonetheless, it has been difficult sometimes, in this method of using the pressure, to dissolve a sufficient amount of oxygen at short time.
As the other problem, there has been a difficulty in carrying out a high-rate charge when the pressured oxygen is sealed inside a battery case. As shown in the above-illustrated reaction (4), an air cathode generates oxygen at a charge. When the pressured oxygen is sealed inside a battery case, the partial pressure inside the battery case remains high, the above-illustrated reaction (4) is less likely to be caused, so that the a high-rate charge becomes difficult to carry out.
Patent Document 4 discloses a metal/oxygen battery, wherein oxygen is condensed by an oxygen condenser, and the battery comprises a means to supply the high-purity oxygen to an anode. This intends to increase a high power by supplying the condensed oxygen according to an output current. Further, Patent Document 5 discloses a nonaqueous electrolyte air battery comprising a nonaqueous liquid electrolyte in which a carbon dioxide is dissolved (claim 3). This intends to restrain a direct oxidation of an anode by dissolving carbon dioxide into the nonaqueous liquid electrolyte and thereby to improve cycle properties.
Patent Document 1: Japanese Patent Application Publication (JP-A) No. 2004-119278
Patent Document 2: Japanese Patent No. 3,515,492
Patent Document 3: Japanese Patent No. 3,764,623
Patent Document 4: JP-A No. 2002-516474
Patent Document 5: JP-A No. 2003-7357